1. Field of the Invention
The invention relates to a system and method for directly generating high pressure hydrogen (compressed hydrogen) required for the utilization of hydrogen energy without using any mechanical pressurizing device, such as a compressor, whereby pure water, such as deionized water, distilled water and purified water after filtration are electrolyzed using a polyelectrolyte membrane (referred to as PEM hereinafter). The invention belongs to technology related to clean hydrogen energy.
2. Description of the Related Art
Carbon dioxide released by using fossil fuels, such as coal and petroleum, in recent years are thought to be major causes of global greenhouse effects. In addition, acid rain caused by nitrogen oxides and sulfur oxides discharged by the combustion of the fossil fuels serves as a major cause of the loss of human health and destruction of forests. Furthermore, there exist fundamental problems that the estimated amount of fossil fuel deposits is limited, and they may be depleted sooner or later.
To suppress these problems from occurring, the development of novel technologies is urgently desired, whereby the consumption of the fossil fuels is depressed or brought to an end, and clean natural energies that are able to be regenerated can be substituted for the fossil fuels are utilized.
The most abundant natural energy, as the substitute of fossil energy, is solar energy. The energy that the earth receives in one hour from the sun corresponds to or exceeds the energy consumed by humankind for one year. It is not a dream to cover the total energy demand of humankind by the solar energy alone, and many technologies for utilizing solar energy, such as solar generators have been proposed.
In the representative well known in the art methods for utilizing natural energies, such as solar generators, aerogenerators and hydroelectric generators, natural energy is taken out and utilized as electric power.
It is difficult to store and transport electric power itself. Electric power is usually stored by charging a battery. However, batteries are heavy, and the charge is consumed by self-discharge during storage while it is not used.
The most crucial problem for the future energy is to be free from the problems as described above, or the energy should be easily stored and transported while being able to be commonly used where and when necessary. Hydrogen is a candidate for generating energy that satisfies the conditions as described above.
Hydrogen can be readily stored, is able to regenerate its energy as electric power, is convenient and efficient as an energy source. Accordingly, it is contemplated to efficiently convert electric power obtained by natural energy into clean hydrogen energy by electrolysis of water, and to use a hydrogen energy source as a substitute of conventional energy sources, such as petroleum. It is anticipated that a hydrogen economy society using hydrogen as the energy source would be realized in the 21st century.
To realize the hydrogen economy society, the development of fuel cells using hydrogen as a fuel (Polyelectrolyte Fuel Cell, abbreviated as PEFC hereinafter) have been actively developed as means for efficiently utilizing hydrogen as the energy source. In addition, uses of hydrogen for automobile and home generators have been also considered. The hydrogen economy society with no anxiety of the greenhouse effect by carbon dioxide would be realized when the methods as described above are spread to enable hydrogen generated by natural energy to be widely utilized.
Such a society as described above is based on an assumption that crucial problems for efficiently generating hydrogen are solved using natural energies, particularly solar energy.
The most important problem for utilizing hydrogen as the energy source is how hydrogen gas could be safely transported and stored in a compact vessel.
For solving the problems as described above, it has been attempted to convert hydrogen gas into liquid hydrogen or to allow hydrogen to be occluded in an occlusive alloy. However, these methods involve unsolvable problems of spontaneous evaporation and insufficient occlusion volume. Since lightweight and highly pressure resistant gas cylinders have been developed in recent years, safety of high pressure hydrogen is reevaluated. Consequently, the hydrogen is often stored and transported by filling in a gas cylinder as compressed hydrogen with a pressure of as high as 350 atm or more. Such a method is widely noticed as a technology compatible for the hydrogen economic society.
When hydrogen is used for fuel cell vehicles using the fuel cells as described above, compressed hydrogen at a pressure of as high as 350 atom should be used. Otherwise, the volume of the hydrogen gas cylinder is large, and the space for the passenger cabin is reduced. When the volume of the hydrogen gas cylinder is small, on the other hand, the cruising distance is so shortened that it is impractical. Accordingly, it is a key point for shifting the current society to the hydrogen economy society to convert hydrogen, utilized as the energy source, into highly compressed hydrogen with a pressure of as high as 350 atm or more.
While hydrogen has been generated by electrolysis of an aqueous alkaline solution prepared by dissolving an alkaline electrolyte such as potassium hydroxide (KOH) in water for a long period of time, electrolysis using a polyelectrolyte membrane (abbreviated as PEM electrolysis hereinafter) by which pure water is directly electrolyzed into hydrogen and oxygen has been noticed in recent years as a result of developments of the polyelectrolyte fuel cells (PEFC). In PEFC water is electrolyzed by a reversed reaction of PEFC using the PEM.
As widely known in the art, since an alkali, such as potassium hydroxide, forms accumulated substances on electrodes by a reaction of the alkali with the impurities, such as carbon dioxide dissolved in water in electrolysis of the aqueous alkaline solution, aqueous electrolysis cells should be periodically cleaned to remove the accumulated substances. A purification device for removing alkali mists generated together with hydrogen is also required.
Since the hydrogen and oxygen generated are separated from each other with a porous partition membrane, such as a gas-permeable asbestos, the mixing ratio between them increases with a decrease in the amount of the generated gas, and the proportion of hydrogen or oxygen permeating through the porous membrane is relatively increased. Consequently, the mixed gas becomes a detonating gas that involves a danger of explosion making it difficult to arbitrarily stop and start gas generation. It is not easy to generate hydrogen by electrolysis of the aqueous alkaline solution using electric power generated by sunlight or aerodynamic power that frequently varies. Furthermore, since the pressure of hydrogen generated by electrolysis of the aqueous alkaline solution is low, use of a gas compressor is required in order to prepare a highly compressed hydrogen.
In contrast, pure water is directly electrolyzed to obtain highly pure hydrogen while hydrogen and oxygen are separated with PEM that permeates only protons in the PEM electrolysis method. Therefore, hydrogen and oxygen are not mixed with each other when electrolysis is suddenly stopped as in electrolysis of the aqueous alkaline, and start and stop of electrolysis may be arbitrarily repeated. Consequently, the PEM electrolysis method is excellent for converting the frequently varying electric power generated by natural energy into hydrogen.
The method for generating high pressure hydrogen by PEM electrolysis is inherently able to generate high pressure hydrogen and oxygen because of conversion of liquid to gas. Namely, a small volume is converted to a large volume with no mechanical pressure increasing device, such as a compressor used in principle. Eventually, hydrogen with a pressure of as high as 1000 atm or more may be obtained by only electrolysis. Since no mechanically movable parts are involved as compared with devices that mechanically increase pressure, periodic maintenance work with frequent inspection and replacement of expendables is not needed. Therefore, maintenance-free and unattended automatic operation for a long period of time is possible enabling the practical conversion of natural energy into hydrogen. Furthermore, since the PEM electrolysis method has a higher compression efficiency as compared with the method using mechanical pressure-increasing devices, such as a compressor, it is an advantage of the PEM electrolysis method that less compression power is required, and much expectation is concentrated on the generation of high pressure hydrogen by PEM electrolysis for energy conversion.
The system for generating hydrogen by PEM electrolysis comprises electrolysis cells prepared by laminating a plurality of unit cells with a structure in which the PEM, having catalytic electrodes such as platinum formed on both surfaces thereof, is sandwiched with the porous electrode through which pure water and gases are permeable. Since each cell is laminated in the electrolysis cell having the structure as described above, the electrode partitioning of each unit cell is a double-polarity electrode because the electrode serves as a cathode as well as an anode. The PEM electrolysis cell comprising laminated unit cells may be called a double-polarity multi-layered type electrolysis cell. Much expectation is concentrated on the emergence of a system for generating high pressure hydrogen by double-polarity multi-layered type electrolysis cells using PEM.
However, it is a current problem of electrolysis by electrolysis cell using PEM that the pressure resistance of the seal member and PEM of the electrolysis cell is as low as about 4 atm. Hydrogen and oxygen gases with a pressure of only several to several tens of atm at most may be generated in the electrolyte cell as described above, and hydrogen with a pressure of as high as 350 atm or more required for energy conversion cannot be generated. Therefore, hydrogen is required to be compressed using a gas compressor for efficient storage and transportation.
For obtaining high pressure hydrogen without using a gas compressor, liquid hydrogen is evaporated to convert it into the high pressure hydrogen, and the hydrogen is charged into a gas cylinder. However, it is a disadvantageous method, because the liquefaction of hydrogen needs a large amount of energy and liquid hydrogen diminishes under transportation and storage by evaporation. Moreover the liquefier needs regular or frequent maintenance, and is hard to produce liquid hydrogen at a remote area under automatic operation with a shortage of hands.
With respect to energy loss, the energy conversion efficiency is decreased in the production of liquid hydrogen as compared to the use of compressed hydrogen since much energy is required in the former case. While about three hundred million cubic meters of hydrogen is sold annually in this country, several tenfolds of hydrogen is estimated to be consumed when only ten percent of domestic automobiles use hydrogen as the fuel. An amount of energy exceeding the amount of hydrogen energy currently available in the market may be consumed as the energy required for liquefying such a vast amount of hydrogen.
Although enough liquefying machines for liquefying such a vast amount of hydrogen should be constructed, the additionally constructed liquefying machines only consume energy without creating additional energy.
Therefore, use of liquefied hydrogen as an energy source is disadvantageous with respect to the energy conversion efficiency, and facilities that do not create additional energy are forced to be constructed to realize the use of liquefied hydrogen.
Accordingly, use of liquefied hydrogen as a high pressure hydrogen source, or as an energy source, is restrictive, and it is hardly conjectured that liquid hydrogen is the major energy source in the hydrogen economy society in the future.
The gas compressor involves, on the other hand, the problems of wear of parts as described previously. Moreover, mechanical pressure increasing devices such as the gas compressor for generating the high pressure hydrogen with a pressure as high as 350 atm or more is a theme of development. Devices with satisfactory functions are not available today. For example commercialized reciprocative compressors cannot make gas over 200 atm and diaphragm compressors need to exchange diaphragms every 1000 hours and its production capacity is 30 N/m3 at most. There are no gas compressors with a capacity of 300 N/m3 and contamination of hydrogen by the gas compressor itself is another problem that cannot be ignored.
When the purity of hydrogen used as the fuel for converting hydrogen into electric power using the PEM fuel cell is poor, the electrodes are poisoned and decrease the output power of the cell, shortening the service life of the cell. Therefore, contamination of hydrogen is a fatal drawback.
The most efficient utilization of energy as the major energy source is accomplished by compressed hydrogen by which the volume of the hydrogen is compressed under a high pressure to enable the hydrogen to be readily stored and transported. The hydrogen can be used as a substitute of the fossil fuels when hydrogen used as an energy source is converted into high pressure hydrogen by reducing its volume for the convenience of storage and transport. Various methods of PEM electrolysis have been studied as suitable methods for generating the high pressure hydrogen by only electrolysis without using a gas compressor. Various methods have been proposed with respect to the device for generating high pressure hydrogen required for utilizing hydrogen as an energy source by only electrolysis, particularly for solving the problem of low pressure resistance of the electrolysis cell.
For example, it was noticed in Japanese Patent Publication No. 3,220,607 (U.S. Pat. No. 5,690,797) that the force acting on the PEM of the double-polarity multi-layered type electrolysis cell is a differential pressure between hydrogen generated in the cathode and oxygen generated in the anode, and that the force acting on the seal member of the cell is a differential pressure between the combined pressure of hydrogen and oxygen in the cell and external pressure of the cell. Therefore, the cell is submerged in pure water in the high pressure vessel for storing pure water and oxygen in order to control the pressure in the high pressure vessel for storing hydrogen and the pressure of the high pressure vessel for storing oxygen to be equal. The differential pressures acting on the PEM and seal member of the cell are controlled within the pressure resistance of the cell. Consequently, only a differential pressure within the pressure resistance of the cell acts on the cell even when hydrogen and oxygen is generated at a combined pressure exceeding the pressure resistance of the cell, thereby enabling high pressure hydrogen to be generated.
However, corrosion of metallic parts should be considered in the device for generating the hydrogen and oxygen gases. The electrolysis cell is submerged in pure water by housing it in the high pressure vessel while storing oxygen generated at the anode in the high pressure vessel. Therefore, the electrolysis cell having the electrodes is sealed in an environment containing high pressure oxygen, that readily causes corrosion of metals, and water together as the pressure is increased.
Furthermore, corrosion of the metallic parts, such as the electrodes, are liable to occur as the temperature is increased in the permissible range of heat resistance of PEM. In addition, the leak current cannot be ignored since the resistivity of pure water in which the PEM electrolysis cell is submerged decreases. When the problem of temperature increase is solved by cooling pure water in which the PEM electrolysis cell is submerged by using a heat exchanger, the cell is forced to be operated at a temperature of 40° C. or less where the cell efficiency becomes poor, and the operating condition is disadvantageous for effective utilization of heat.
Therefore, this proposal involves inherent problems to be solved such as electrolytic corrosion by oxygen and leak electric current by the decrease of resistivity of pure water, in order to generate high pressure hydrogen required for utilizing hydrogen as an energy source.
When abnormalities, such as a break of PEM isolating the anode compartment of the electrolysis cell from its cathode compartment, or a break of the seal member of the electrolysis cell occur, a large amount of hydrogen is mixed with oxygen in the high pressure cell housing the electrolysis cell, arising a danger of generating a detonating gas. Therefore, a countermeasure for this danger is also required.
Accordingly, while the generation of high pressure hydrogen with a pressure of as high as several hundreds of atm or more is possible in principle in this device for generating hydrogen and oxygen, the device is currently only applicable for generating hydrogen with a pressure of several tens of atm, and it is not easy to generate high pressure hydrogen with a pressure of several hundreds atm that is considered necessary for utilizing hydrogen as an energy source.
A part of the electric current flowing in the electrolysis cell flows in pure water in which the electrolysis cell is submerged by the decrease of resistivity of pure water, even when the problem of corrosion of metals is solved, thereby decreasing electrolysis efficiency due to electric power loss. Moreover, since pressure resistance and heat resistance of the ion-exchange resin are low, another problem is that the decreased resistivity as a result of the decreased purity of pure water in the high pressure vessel cannot resume its original high resistivity by regenerating contaminated pure water into pure water using an ion-exchange resin. In particular, this is a serious problem because the electrolysis efficiency is enhanced by increasing the temperature to about 80° C. or more.
While pure water should always be regenerated with the ion-exchange resin due to accelerated dissolution of wall substances of the vessel into pure water when the temperature of pure water is increased for decreasing resistivity, the pressure of the cell is restricted because the ion-exchange resin is broken by treating pure water under a high pressure. Consequently, it was difficult to generate high pressure hydrogen required for utilization of hydrogen as a energy source.
For solving these problems, Japanese Unexamined Patent Application Publication No. 2001-130901 has proposed a hydrogen energy feed device constructed so that electrical insulation is not compromised even at a high electrolysis temperature, wherein hydrogen and oxygen generated by electrolysis are stored in separate high pressure tanks while hermetically immersing the electrolysis cell in an electrically insulating liquid in an exclusive high pressure vessel in order to prevent corrosion of metals, such as the electrode, due to coexistence of oxygen and water at a high temperature and pressure.
This method not only settles both problems of corrosion by electrolysis and decrease of resistivity of pure water at once, but is also able to prevent the detonating gas from being generated since pure water serves to isolate oxygen from hydrogen even when the electrolysis cell is broken, thereby greatly improving safety of the cell.
However, this method is still difficult to practically employ since no practically available electrically insulating liquid for immersing the electrolysis cell in the high pressure vessel has not been found yet.
A vast quantity of electrically insulating liquid is needed for covering the demands of the device for generating enough hydrogen to be converted into the vast amount of energy that is supposed to be consumed. However, it is difficult to chemically synthesize and use a large quantity of the electrically insulating liquid without any burden to the environment, or so that the environment, particularly groundwater and soil, is not readily polluted by leakage. Moreover, the liquid is required to be incombustible and chemically stable so that the liquid is not reactive with a minute quantity of oxygen and hydrogen leaking from the electrolysis cell while having no danger of explosion by reacting with oxygen even when a large quantity of oxygen is leaked in the high pressure vessel. Such electrically insulating liquids that satisfies these conditions have not been found.
For example, although PCB is a flame-retarded liquid with excellent performances, its production and use are forbidden from the view point of public hazard and environmental pollution. Therefore, all the currently available insulating oils are inflammable, and involve a potential danger of explosion when oxygen is leaked.
In addition, pure water is difficult to use since the resistivity of pure water changes with time, as described above, although pure water itself is excellent as a insulating liquid.
Since pure water has a potential to dissolve all the substances, the resistivity of pure water is gradually decreased when pure water is sealed in the high pressure vessel. This decrease of resistivity not only decreases efficiency of the cell due to a leak electric current generated, but also hydrogen and oxygen are generated by the leak electric power in the high pressure vessel housing the electrolysis cell increasing the pressure. This increase of the pressure may cause a potential danger by which the electrolysis cell may be finally crushed by the pressure, or the mixed gas of hydrogen and oxygen may explode. Therefore, countermeasures for these potential dangers should be provided.